1. Field of the Invention
The present invention relates to display devices, and more particularly to plasma discharge display devices. Still more particularly, the present invention relates to apparatus and methods for generating one- and two-dimensional scanned plasma discharge displays having continuously variable brightness levels.
2. Prior Art
Recent years have witnessed the greatly increased use of arrays of plasma discharge devices for generating data displays. For example, U.S. Pat. No. 3,559,190 issued to D. L. Bitzer et al. on Jan. 26, 1971 and U.S. Pat. No. 3,499,167 issued to T. C. Baker et al. on Mar. 3, 1970 illustrate a typical form of plasma display device, commonly called the plasma panel. Standard plasma panel configurations include a matrix array of plasma cells defined by the overlap of respective pluralities of horizontal and vertical (row and column) electrodes. By using standard half-select addressing techniques, it is possible to write or erase any particular plasma cell. The advantages of such plasma panel systems follow in large part from the self-memory characteristic of the plasma cells. Thus, while information-bearing signals are necessary to write and erase information in particular cells, no additional information is required to maintain a display once established. Rather, only periodic non-information-bearing sustain signals are required.
One disadvantage of the most common plasma panel configurations is that, in general, it is possible only to display binary information, i.e., "on" or "off" conditions of plasma cells. Thus, the potential for displaying information of varying intensity such as might be associated with standard television displays, has not generally been realized using standard plasma panels.
Some attempts have, however, been made to generate gray level displays using modified arrangements of standard plasma panel structures. For example, Ngo, "A Multiple Intensity Plasma Display with Internal Memory," Proceedings of the S.I.D., Vol. 13, No. 1, first quarter 1972, pages 21-25 illustrates one technique for generating gray scale in a plasma panel system. See also, U.S. Pat. No. 3,626,241 issued Dec. 7, 1971 to Ngo. The complexity involved in generating the multiple images described in the last-mentioned paper is, however, not justified in many applications. Thus, for example, approximately 50 percent more drive circuitry (as compared with standard panels) is typically required for the gray scale plasma panel system described in the above-cited Ngo paper. When it is considered that a major objection to present two-level (on-off) plasma panels is the need for rather complex drive circuitry, it can be appreciated that any additional complexity will be tolerated only with great reluctance.
Other attempts to introduce gray scale operation of a plasma panel include that described in U.S. Pat. No. 3,735,182 issued May 22, 1973 to J. J. Dalton, et al. which requires additional conductor patterns, each having a different resolution. U.S. Pat. No. 3,742,483 issued June 26, 1973 to J. A. Ogle; U.S. Pat. No. 3,654,388 issued Apr. 4, 1972 to H. G. Slottow, et al.; and U.S. Pat. No. 3,778,673 issued Dec. 11, 1973 to M. E. Eisenberg, et al. describe a plasma panel including means for modulating the amplitude and/or duration of cell current, thereby varying cell brightness. Each of these described systems requires at least a degree of complexity in panel fabrication and drive circuitry equal to that for bilevel (on/off) plasma panels.
While much emphasis has been placed in prior art plasma panel systems on the matrix arrangement of rows and columns of individual plasma cells, some effort has been directed to producing more integral structures. Thus, for example, in U.S. Pat. No. 3,781,600 issued Dec. 25, 1973 to W. E. Coleman et al., there is described a plasma device for transferring a discharge linearly along a prescribed path, i.e., in a manner analogous to transferring signals representative of digital information in a linear shift register. The structure described in the Coleman et al. patent has been generalized to a two-dimensional structure as described in U.S. Pat. No. 3,775,764 issued Nov. 27, 1973 to J. P. Gauer. Another plasma discharge shifting device is described in U.S. Pat. No. 3,795,908 issued Mar. 5, 1974 to A. W. McDowell et al.
The structure described in the last-mentioned Coleman et al., Gauer, and McDowell et al. patents will be seen to share with plasma panels of the type described in the earlier-mentioned Bitzer et al. and Baker et al. patents, the characteristic of individual pluralities of location-defining electrodes positioned on opposite sides of a cavity containing an illuminable inert gas. One important limitation of all such two-sided structures is the need for accurate registration of cell-defining electrodes to ensure the establishment and/or transfer of a discharge at the respective cells. If rather close tolerances in physical structure are not observed, undue burdens are placed on the requirements for the various drive and sustain signals. Inability to reliably reproduce physical structure and electrical characteristics of various of the plasma devices using the multilevel "sandwich" panel construction has resulted in low manufacturing yield and attendant high cost.
One solution for improving the reproducibility of structures for realizing plasma discharge sites is described in U.S. Pat. Nos. 3,646,384, 3,666,981, and 3,719,940 issued to F. M. Lay on Feb. 29, 1972, May 30, 1972, and Mar. 6, 1973, respectively. The structure described in the Lay patents is essentially planar, thereby avoiding many of the alignment and registration problems inherent in sandwich-type panels. An improved version of the single substrate configuration is described in U.S. patent application Ser. No. 444,380 filed Feb. 21, 1974 by G. W. Dick et al. The structure described in the Dick et al. application and the various Lay patents is also characterized by selection of particular cells by coincident current techniques.
In U.S. Pat. No. 3,798,502 issued to P. D. Ngo on Mar. 19, 1974 (hereinafter the '502 patent), there is disclosed a planar gas discharge shift register having simplified gas cell definition and greatly facilitated discharge transfer characteristics. Copending U.S. patent applications by P. D. Ngo, Ser. No. 520,613 entitled "Plasma Discharge Shift Registers," and Ser. No. 520,614 entitled "Planar Raster Scan Display Having Gas Discharge Shift Registers," both filed of even date herewith (hereinafter the Ngo applications) describe further modifications to the invention covered by the '502 patent. To simplify the present disclosure, both the '502 patent and the Ngo applications are hereby incorporated by reference in the present application.
While improved plasma shift register structures are disclosed in the '502 patent and the Ngo applications, only the last-cited Ngo application provides explicitly for a two-dimensional interconnection to provide TV-like displays. Further, only the same last-cited Ngo application teaches the incorporation in a shift register structure of a gray scale capability such as is required for standard television or similar displays.
It is therefore an object of the present invention to provide a shift register-based display system capable of generating and maintaining one- and two-dimensional images.
It is a further object of the present invention to provide such a display system having a gray scale capability.